The catalytic epoxidation of ethylene using a silver-based catalyst has been practiced for a long time. However, conventional silver-based catalysts have provided ethylene oxide notoriously in a low selectivity. For example, when using conventional catalysts, the selectivity towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 mole-% limit. Therefore, this limit has long been considered to be the theoretically maximal selectivity of this reaction, based on the stoichiometry of the reaction equation7C2H4+6O2=>6C2H4O+2CO2+2H2O,cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd ed., Vol. 9, 1980, p. 445.
Modern silver-based catalysts however are highly selective towards ethylene oxide production. When using the modern catalysts in the epoxidation of ethylene the selectivity towards ethylene oxide can reach values above the 6/7 or 85.7 mole-% limit referred to. Such highly selective catalysts may comprise as their active components silver, and one or more dopants, such as rhenium, tungsten or molybdenum or a nitrate- or nitrite-forming compound, or components comprising rhenium, tungsten or molybdenum or a nitrate- or nitrite-forming compound. Frequently, the high selectivity catalysts comprise as additional dopants one or more Group IA metals, or one or more components comprising Group IA metals. Preferred Group IA metals are the higher Group IA metals having an atomic number of at least 37, for example rubidium and, in particular, cesium. The Group IA metals having an atomic number of at least 37 may hereinafter be referred to by the term “higher Group IA metals”. Highly selective catalysts are disclosed, for example, in U.S. Pat. No. 4,761,394 and U.S. Pat. No. 4,766,105, and in several subsequent patent publications.
The highly selective catalysts are in particular subject to an aging-related performance decline during normal operation and they tend to be exchanged more frequently than the conventional catalysts. The aging manifests itself by a reduction in the activity of the catalyst. Usually, when a reduction in activity of the catalyst is manifest, the reaction temperature is increased in order to compensate for the reduction in activity. The reaction temperature may be increased until it becomes undesirably high, at which point in time the catalyst is deemed to be at the end of its lifetime and would need to be exchanged. It goes without saying that from an economical point of view it is highly desirable to extend the lifetime of the catalyst as much as possible.